Glass for eye glass lens

ABSTRACT

A glass for an eye glass lens comprising a SiO 2  (or B 2  O 3 )-CaO-Nb 2  O 5  system as a basic system and having a refractive index of 1.79 or more, an Abbe number of 32 to 38 and a specific gravity of less than 3.6 is disclosed.

FIELD OF THE INVENTION

The present invention relates to a glass composition for an eye glass lens having a refractive index (nd) of 1.79 or more, an Abbe number (νd) of 32 to 38 and a specific gravity of less than 3.6.

BACKGROUND OF THE INVENTION

The correction of eye glass lenses depends on the refractive index and the curvatures of both surfaces of the lens but when the degree of myodia or hypermetropia advances, the thickness of the lens for correcting it increases at the outer edge portion or the center portion and hence the appearance is spoiled and the lens becomes heavy in weight to give unpleasant feeling to the user. This problem can be improved by increasing the refractive index of glass material. However, if the refractive index of the glass increases, the specific gravity of the glass generally tends to increase. Therefore, a glass having a high refractive index and a low specific gravity has been demanded and such a glass is disclosed in, for example, Japanese Patent Publication No. 51141/81 and Japanese Patent Application (OPI) No. 126549/80. However, the glass disclosed therein contains a large amount of TiO₂ to obtain a low specific gravity and, as a result, has a small Abbe number.

Therefore, the glass has the disadvantage that the chromatic aberration at the outer edge portion thereof is large when the glass is used as an eye glass lens. In particular, when the glass is used as an eye glass lens having a high diopter value, the chromatic aberration becomes a serious problem.

For the above reasons, a light weight glass having a high refractive index and simultaneously a large Abbe number is demanded.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a glass for an eye glass lens having a refractive index (nd) of 1.79 or more, a relatively large Abbe number (νd) of 32 to 38 and a specific gravity of less than 3.6.

The glass according to the present invention comprises, in % by weight,

22 to 36% SiO₂ +B₂ O₃ ;

0 to 36% SiO₂ ;

0 to 36% B₂ O₃ ;

16 to 42% CaO+MgO;

8 to 42% CaO;

0 to 22% MgO;

14 to 36% TiO₂ +Nb₂ O₃ ;

0 to 19% TiO₂ ;

1 to 35% Nb₂ O₅ ;

0 to 13% ZrO₂ ;

0 to 14% La₂ O₃ ;

0 to 17% Y₂ O₃ ;

0 to 10% Gd₂ O₃ ;

0 to 10% Ta₂ O₅ ;

0 to 12% WO₃ ;

0 to less than 5% Li₂ O+Na₂ O+K₂ O;

0 to 14% BaO;

0 to 16% SrO;

0 to 14% ZnO;

0 to 14% Al₂ O₃ ; and

0 to 10% P₂ O₅,

wherein the total amount of the trivalent or more components is at least 1.05×(SiO₂ +B₂ O₃ +Al₂ O₃ +P₂ O₅ +alkali metal oxides).

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that a SiO₂ (or B₂ O₃)-CaO-Nb₂ O₅ system glass as a basic system wherein the ratio between the trivalent or more oxide which is the component to increase the refractive index and the glass former and alkali metal oxide which are the component to decrease the refractive index is strictly limited and also the component which increases the Abbe number and the component which increases the specific gravity are limited to the specific ranges is suitable for the object of the present invention.

SiO₂ and B₂ O₃ are the components comprising the skeleton of the glass. If the total amount thereof is higher than 36% by weight, the desired high refractive index cannot be obtained and on the other hand, if the total amount thereof is less than 22% by weight, a glass having the desired low weight cannot be obtained.

CaO is the component necessary to increase the stability of the glass toward devitrification and must be present in an amount of at least 8% by weight. However, if the amount thereof is higher than 42% by weight, the glass shows a strong tendency toward devitrification.

MgO can be substituted with CaO without increasing the specific gravity of the glass. If the amount of MgO is higher than 22% by weight, a devitrification tendency of the glass increases.

The total amount of MgO and CaO must be within the range of from 16 to 42% by weight.

TiO₂ and Nb₂ O₅ are the component useful to obtain a glass having a high refractive index and a low specific gravity and the total amount thereof must be at least 14% by weight. However, due to the disadvantage of those components that an Abbe number of a glass is decreased, the amount of TiO₂ must be limited to 19% by weight or less, the amount of Nb₂ O₅ must be limited to 35% by weight or less and the total amount thereof must be limited to 36% by weight or less. Further, Nb₂ O₅ is the essential component and must be present in an amount of 1% by weight or more to prevent devitrification of a glass.

Other components are not necessarily essential and are added for the purpose of improving meltability in batch, devitrification resistance or moldability and adjusting optical constants.

However, if the amounts of ZrO₂, an alkali metal oxide, Al₂ O₃ and P₂ O₅ are higher than 13% by weight, 5% by weight, 14% by weight and 10% by weight, respectively, a devitrification of a glass rapidly deteriorates.

BaO, SrO, ZnO, La₂ O₃, Y₂ O₃, Gd₂ O₃, Ta₂ O₅ and WO₃ which are the component to increase a specific gravity of a glass, should not be added in amounts higher than 16% by weight, 14% by weight, 15% by weight, 17% by weight, 10% by weight, 10% by weight and 12% by weight, respectively.

The total amount of trivalent or more components must be 1.05×(SiO₂ +B₂ O₃ +Al₂ O₃ +P₂ O₅ +alkali metal oxide) or more.

Small amounts of As₂ O₃, Sb₂ O₃, F₂, GeO₂ and PbO can be added to the glass composition of the present invention to improve purification and meltability in a range which does not degrade the characteristics of the glass.

According to the preferred embodiment of the present invention, the total amount of SiO₂ and B₂ O₃ is limited to 33% by weight or less to obtain a glass having a high refractive index and 26% by weight or more to obtain a glass having a low specific gravity. However, in view of the fact that a chemical durability of a glass deteriorates with increasing the proportion of B₂ O₃, it is preferred that the amount of B₂ O₃ is 0 to 15% by weight and the amount of SiO₂ is 15 to 33% by weight.

To obtain a glass having a particularly excellent devitrification resistance, the total amount of CaO+MgO and the total amount of TiO₂ +Nb₂ O₅ each is 20 to 30% by weight, with proviso of 12 to 36% by weight CaO, 0 to 15% by weight MgO, 4 to 16% by weight TiO₂ and 4 to 30% by weight Nb₂ O₅.

ZrO₂ has a large devitrification preventing effect and is effective to provide a glass of a low dispersion as compared with TiO₂ and Nb₂ O₅. The preferred amount of ZrO₂ is 2 to 10% by weight.

La₂ O₃ and Y₂ O₃ are effective to provide a glass of a low dispersion as compared with TiO₂, Nb₂ O₅ and ZrO₂ and are the component advantageous to obtain a glass having a large Abbe number. On the other hand, those components increase a specific gravity of a glass. It is therefore preferred that the amount of La₂ O₃ is 0 to 10% by weight, the amount of Y₂ O₃ is 0 to 14% by weight and the total amount thereof is 0 to 14% by weight.

Li₂ O, Na₂ O and K₂ O are effective to improve a meltability of a glass ih batch and it is preferred from the standpoint of a devitrification resistance that the total amount thereof is 4% by weight or less.

BaO, SrO and ZnO are effective to improve a devitrification resistance but each increases a specific gravity of a glass. Therefore, it is preferred that the total amount thereof is 10% by weight.

It is preferred that the total amount of trivalent or more components is at least 1.05×(SiO₂ +B₂ O₃ +alkali metal oxide) to maintain the desired high refractive index.

Accordingly, the most advantageous embodiment of the present invention is a glass composition having a refractive index (nd) of 1.795 or more, an Abbe number (νd) of 32 to 36 and a specific gravity of 3.55 or less, which comprises, in % by weight,

26 to 36% SiO₂ +B₂ O₃ ;

15 to 33% SiO₂ ;

0 to 15% B₂ O₃ ;

20 to 36% CaO+MgO;

12 to 36% CaO;

0 to 15% MgO;

20 to 36% TiO₂ +Nb₂ O₅ ;

4 to 16% TiO₂ ;

4 to 30% Nb₂ O₅ ;

2 to 10% ZrO₂ ;

0 to 14% La₂ O₃ +Y₂ O₃ ;

0 to 10% La₂ O₃ ;

0 to 14% Y₂ O₃ ;

0 to 4% Li₂ O+Na₂ O+K₂ O; and

0 to 10% BaO+SrO+ZnO

wherein the total amount of the trivalent or more oxides is at least 1.05×(SiO₂ +B₂ O₃ +alkali metal oxides).

Examples of the glass compositions of the present invention are shown in the following table together with the properties thereof, wherein the components are shown by weight %.

                                      TABLE 1                                      __________________________________________________________________________     No   1  2  3  4  5  6  7  8  9  10  11 12 13 14 15 16 17 18 19                 __________________________________________________________________________     SiO.sub.2                                                                           30.1  22.3                                                                              12.6                                                                              20.6                                                                              23.7                                                                              25.6                                                                              23.7                                                                              27.6                                                                              22.3                                                                               22.3                                                                              25.6                                                                              22.6                                                                              22.0                                                                              22.3                                                                              21.3                                                                              27.6                                                                              24.6                                                                              12.6               B.sub.2 O.sub.3                                                                        31.7                                                                              6.0                                                                               19.1                                                                              4.1   4.0                                                                               4.0                                                                               4.1                                                                               7.0 7.0                                                                               4.3                                                                               8.0                                                                               8.0                                                                               8.0                                                                               8.0                                                                               4.1                                                                               4.1 16.1              CaO  25.5                                                                              28.4                                                                              19.9                                                                              28.4                                                                              20.4                                                                              30.6                                                                              28.4                                                                              28.4                                                                              26.4                                                                              22.9                                                                               22.9                                                                              28.1                                                                              33.6                                                                              33.6                                                                              34.9                                                                              38.4                                                                              28.4                                                                              10.4                                                                           31.4                  MgO        15.0                                          13.0                  TiO.sub.2                                                                           10.5                                                                              4.9                                                                               15.4                                                                              4.9                                                                               11.0                                                                              11.0                                                                              11.0                                                                              11.0                                                                              7.0                                                                               15.4                                                                               15.4                                                                              11.0                                                                              14.0                                                                              14.0                                                                              15.4                                                                              11.4                                                                              4.9                                                                               7.0                   Nb.sub.2 O.sub.5                                                                    16.5                                                                              28.2                                                                              7.6                                                                               28.2                                                                              16.0                                                                              16.0                                                                              16.8                                                                              16.0                                                                              24.0                                                                              7.6 7.6                                                                               5.0                                                                               10.0                                                                              10.0                                                                              7.6                                                                               15.6                                                                              28.2                                                                              24.0                                                                           28.2                  Al.sub.2 O.sub.5 7.0                                                           P.sub.2 O.sub.5     5.8                                                        ZrO.sub.2                                                                           9.8                                                                               2.0                                                                               7.8   6.8                                                                               7.8                                                                               6.0                                                                               6.8                                                                               6.8                                                                               7.8 7.8                                                                               6.8                                                                               1.8                                                                               2.4                                                                               7.8                                                                               5.3                                                                               6.8                                                                               6.8 6.0               Li.sub.2 O                                                                          4.0                                                                       Na.sub.2 O             2.0                                                     K.sub.2 O                 4.0                                                  BaO                             12.0                                           SrO                                 12.0                 6.0                   ZnO              8.0         2.0                         2.0                   La.sub.2 O.sub.3                                                                    3.6                                                                               4.8                                                                               6.0                                                                               6.8                                                                               6.1                                                                               5.1   6.1                                                                               2.1                                                                               5.0 5.0                                                                               7.1                                                                               3.0                                                                               3.0                                                                               4.0      2.1 5.7               Y.sub.2 O.sub.3                        12.1                                    Gd.sub.2 O.sub.3       6.2                                                     Ta.sub.2 O.sub.5                          7.0                                  WO.sub.3                                     7.0                               A*.sup.1                                                                            35.8                                                                              33.3                                                                              29.7                                                                              33.3                                                                              33.3                                                                              31.0                                                                              33.2                                                                              33.3                                                                              33.3                                                                              30.8                                                                               30.8                                                                              31.4                                                                              32.1                                                                              31.5                                                                              31.8                                                                              30.8                                                                              33.3                                                                              30.1                                                                           30.1                  B*.sup.2                                                                            40.4                                                                              39.9                                                                              36.8                                                                              39.9                                                                              39.9                                                                              39.9                                                                              40.0                                                                              39.9                                                                              39.9                                                                              35.8                                                                               35.8                                                                              42.0                                                                              35.8                                                                              36.4                                                                              34.8                                                                              32.3                                                                              39.9                                                                              39.9                                                                           39.9                  ηd                                                                              1.800                                                                             1.805                                                                             1.801                                                                             1.799                                                                             1.817                                                                             1.804                                                                             1.798                                                                             1.793                                                                             1.803                                                                             1.810                                                                              1.806                                                                             1.790                                                                             1.796                                                                             1.797                                                                             1.798                                                                             1.797                                                                             1.800                                                                             1.801                                                                          1.795                 νd                                                                               33.8                                                                              34.8                                                                              33.6                                                                              34.3                                                                              32.2                                                                              33.6                                                                              33.3                                                                              33.5                                                                              33.3                                                                              32.8                                                                               33.1                                                                              37.0                                                                              33.6                                                                              33.2                                                                              33.9                                                                              34.5                                                                              33.6                                                                              33.7                                                                           36.9                  Specific                                                                            3.42                                                                              3.41                                                                              3.44                                                                              3.46                                                                              3.59                                                                              3.50                                                                              3.48                                                                              3.44                                                                              3.48                                                                              3.59                                                                               3.55                                                                              3.56                                                                              3.44                                                                              3.43                                                                              3.36                                                                              3.34                                                                              3.44                                                                              3.59                                                                           3.52                  gravity                                                                        __________________________________________________________________________      *.sup.1 A = Σ (SiO.sub.2 + B.sub.2 O.sub.3 + Al.sub.2 O.sub.3 +          P.sub.2 O.sub.5 + Li.sub.2 O + Na.sub.2 O + K.sub.2 O) × 1.05            *.sup.2 B = Σ (TiO.sub.2 + Nb.sub.2 O.sub.5 + La.sub.2 O.sub.3           Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3 + Ta.sub.2 O.sub.5 + WO.sub.3)        

These glasses of the present invention can be produced by melting a mixture of starting materials which are conventionally used such as siliceous stone powder, boric acid, calcium carbonate, magnesium carbonate, titanium oxide and niobium oxide in a platinum crucible at 1,300° to 1,400° C. followed by stirring to sufficiently homogenize the mixture and remove bubbles, casting the molten mixture into a metallic mold which has been pre-heated at an appropriate temperature, and the annealing the product.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 

What is claimed is:
 1. A glass for an eye glass lens having a refractive index of 1.79 or more, an Abbe number of 32 to 38 and a specific gravity of less than 3.6, consists essentially of, in % by weight,22 to 36% SiO₂ +B₂ O₃ ; 0 to 36% SiO₂ ; 0 to 36% B₂ O₃ ; 16 to 42% CaO+MgO; 8 to 42% CaO; 0 to 22% MgO; 14 to 36% TiO₂ +Nb₂ O₅ ; 0 to 19% TiO₂ ; 1 to 35% Nb₂ O₅ ; 0 to 13% ZrO₂ ; 0 to 14% La₂ O₃ ; 0 to 17% Y₂ O₃ ; 0 to 10% Gd₂ O₃ ; 0 to 10% Ta₂ O₅ ; 0 to 12% WO₃ ; 0 to less than 5% Li₂ O+Na₂ O+K₂ O; 0 to 14% BaO; 0 to 16% SrO; 0 to 14% ZnO; 0 to 14% Al₂ O₃ ; and 0 to 10% P₂ O₅,wherein the total amount of the trivalent or more oxides at least 1.05×(SiO₂ +B₂ O₃ +Al₂ O₃ +P₂ O₅ +alkali metal oxides).
 2. A glass for an eye glass lens as claimed in claim 1, wherein the glass has a refractive index of 1.795 or more, an Abbe number of 32 to 36 and a specific gravity of 3.55 or which consists essentially of, in % by weight,26 to 33% SiO₂ +B₂ O₃ ; 15 to 33% SiO₂ ; 0 to 15% B₂ O₃ ; 20 to 36% CaO+MgO; 12 to 36% CaO; 0 to 15% MgO; 20 to 36% TiO₂ +Nb₂ O₅ ; 4 to 16% TiO₂ ; 4 to 30% Nb₂ O₅ ; 2 to 10% ZrO₂ ; 0 to 14% La₂ O₃ +Y₂ O₃ ; 0 to 10% La₂ O; 0 to 14% Y₂ O₃ ; 0 to 4% Li₂ O+Na₂ O+K₂ O; and 0 to 10% BaO+SrO+ZnO,wherein the total amount of the trivalent or more oxides is at least 1.05×(SiO₂ +B₂ O₃ +alkali metal oxides). 